This invention relates to lasers and, more particularly, to a method and apparatus for phase-locking a plurality of optical waves propagating in a monolithic laser array to cause them to operate as a single coherent source.
During the late 1970's, there evolved a need for III-V semiconductor lasers capable of emitting light powers in the range of 20 to 70 mW. The first generation of these high power devices were the so-called large optical cavity (LOC) lasers. For the most part, they were operated in a pulsed mode. These devices, though capable of meeting the needs of the time, were still limited in how much ultimate power they could deliver. This is because the highest optical power any semiconductor laser can deliver is limited by its ability to withstand the catastrophic mirror damage.
Another class of high power lasers were the facet coated lasers. The outputs of ordinary laser diodes were increased by appropriately coating one facet of the device with anti-reflection coatings. The improvements gained by this method are not very significant in view of the difficulties involved in the deposition of these coatings.
Research then focussed on discrete arrays. In these devices, a series of individual diode lasers are disposed side-by-side and electrically connected in series. The devices are separated from one another by a few millimeters. Optical peak powers in excess of 4 W were achieved from the discrete arrays of this kind, thus satisfying the need for high power. They did not, however, meet the requirement for narrow, collimated beams. Beam forming optics still had to be used to collimate the optical outputs of these devices. This meant that the range of applicability was limited.
The principles of the solution of the problem of obtaining beams with narrow spatial extent in semiconductor lasers are intimately linked to the physics of diffraction in narrow apertures.
The overall size of the ordinary semiconductor laser dictates the spatial extent of the beam emanating from it. In order to circumvent this physical restriction, methods had to be found to increase the lateral effective aperture from wwhich most of the radiation is emitted. It was with this motivation that linear monolithic arrays were developed. If the individual array elements can be phase-locked, narrower beams can be achieved. This follows directly from the physics of diffraction. The first evanescently-coupled linear arrays were studied by Ripper and Paoli circa 1970. These early studies established mutual interaction amongst the elements of the array. The possibilities of phase-locking and beam-steering monolithic laser arrays were shown by Scifres et al. in U.S. Pat. No. 4,255,717 issued Mar. 10, 1981. For the majority of the evanescently-coupled devices reported, operation in a narrow, single-lobe has been more of an exception than the rule. [See D. Botez and D. Ackley, IEEE Circuits and Devices Magazine, 2, 8 (1986)]. This is mainly because there is a lack of control of the phase relationship required for collective coherence of the array elements. The implementation of a deliberate control mechanism for this phase relationship is therefore crucial for the operation of the devices as a single coherent source.